1. Field of the Invention
This invention relates to the elimination of zero point errors in converters which use an operational amplifier for the realization of the analog quantity comparator.
2. The Prior Art
In conventional units, even when the voltage difference between the inputs of the comparator is zero, an output voltage is released which deviates from 0 and thus incorrectly indicates the presence of an inequality of the signals supplied to the two inputs. In the vicinity of the zero point of the characteristic of the analog-digital conversion, such an offset error has the result that very small input signals which are caused by crosstalk, and which can be suppressed due to the quantization in the case of such a code conversion, nevertheless are evaluated as useful signal which has exceeded the first quantization level. Therefore, in the case of imperfect zero point equalization, crosstalk is incompletely suppressed.
It is possible to equalize the zero point errors, with the help of potentiometers at the inputs of the comparators, by which a so-called offset voltage is set which is so large that the output signal is precisely 0, and the offset error is thus compensated. This manner of compensation, because of the required equalization operation, is relatively expensive and is not effective against long-time effects which are caused by aging.
Another known measure for the zero equalization of the comparator in the case of analog-digital converters consists in (when no analog signal which is to be converted is present) forcing, with equal probability, positive and negative signs for the digital signal resulting from the "zero" value then present. However, since in the case of speech generated signals, whereby one is concerned with analog quantities to be encoded, the occurrence of negative and positive signs is not uniformly distributed, there results interfering noises. Besides this, the known method mentioned requires a very precise correspondence of the voltage values which lead to the arising of the two signs forced with equal probability, which requires implicated and expensive control apparatus.